The present invention relates to a new and improved method of and apparatus for cleaning and purifying a liquid contaminated with radioactive substances, wherein the liquid is fed into a evaporator in or at which there is operatively connected a column having a multiplicity of superposed plates or floors, and wherein the vapor generated in the evaporator is guided through a washing liquid uniformly distributed at the floors and flowing in crosswise countercurrent or counterflow with respect to the vapor.
Owing to the requirements regarding protection of the environment is is increasingly necessary to eliminate from liquids contaminated with radioactive substances the contaminants prior to depositing the same in the open waters or waterways. Such purification of the liquid is particularly required in those situations where there is present liquids or waste water charged with radioactive substances. The radioactivity is indicated in microcuries per cm.sup.3 (uCi/cm.sup.3) or in Curies per m.sup.3 (Ci/m.sup.3). According to the generally applicable standards waste water which possesses long-life radioactive substances and should be deposited into the open waterways should possess a radioactivity which is smaller than 10.sup..sup.-7 Ci/m.sup.3.
It is known that in the case of radioactive waste water it is not the water which is the carrier of the radioactivity. Such is quite to the contrary bound to the dissolved salts or suspended solid particles, such as for instance non-dissolved salts, oxides, metal abrasions or dust and the like.
According to a known technique the radioactive waste water is evaporated, and the vapor and the drolets entrained thereby guided through a washing or scrubbing liquid. Consequently, a large portion of the droplets are transferred to the washing liquid. The droplets are admixed with the washing liquid, whereby the radioactive substance contained in the droplets is thinned out or diluted by the washing liquid i.e. concentrated in the washing liquid. As a result the radioactivity per unit of volume, is lowered for instance per cm.sup.3 . Additionally, the droplets entrained by the vapor are separated out in droplet separators which in each case are arranged above the floors. These droplet separators are constructed as demisters and fabricated from pressed mats formed of steel wool or similar material.
Due to the separation of the droplets in such demister there occurs therein an enrichment of contaminants containing radioactive substances, which remain adherent to the fabric of the demister. Consequently, the radioactivity is increased within the column. Due to a simple flushing of this column, i.e. the floors and the demister, experience has shown that there cannot be realized any satisfactory detachment of such radioactive substances.
Furthermore, there are known columns with a multiplicity of floors. Upon these floors there is arranged apart from an inflow weir and an outflow weir, rendering possible a uniform distribution of the liquid at the column floors, an impact plate which is attached above a column floor. This impact plate renders possible deflecting the vapor penetrating through the floors and through the liquid collected at the floors respectively, wherein the droplets entrained by the vapor stream are partially separated out of the vapor stream. Due to this type of droplet separation there is achieved the result that there cannot occur at any location of the column an impermissible enrichment of radioactive substances.
When using these two state-of-the-art techniques there has however been found that owing to the droplet separation itself there cannot be attained the required end concentration in the distillate; this can only be achieved by considerably increasing the quantity of washing liquid. This measure however requires a considerable expenditure in thermal energy inasmuch as the additional washing liquid together with the contaminated liquid must be evaporated. Furthermore it is necessary to enlarge the floor diameter through which there must penetrate the vapor and with it the entrained droplets. Also these measures constitute an increase in the expense of the entire installation.